Problem: A volunteer group is planting trees at $5$ different parks. They planted $90$ trees at each park. How many trees did the group plant in all?
Explanation: ${90}$ trees is the same as ${9\text{ groups of ten}}$ trees. There are $5$ parks. Total number of trees: $\begin{aligned}&5\times{9\text{ groups of ten}}\\ =&{45\text{ groups of ten}}\\ =&{450}\end{aligned}$ The group planted ${450}$ trees.